The present invention relates to an improved process for separating the components of an azeotropic or azeotrope-like mixture containing only organic compounds of which at least one compound is a halocarbon wherein it is difficult and/or impractical to separate the azeotropic or azeotrope-like mixture by conventional means. More specifically, the present invention relates to separating the components of such azeotropic or azeotrope-like mixture by contacting the mixture with a semipermeable membrane to form at least one exit stream having an increased concentration of at least one component and at least one other exit stream having an increased concentration of at least one other component. The exit streams from the contacting step may, thereafter, be recycled for contact with the semipermeable membrane or further processed by conventional means to achieve additional separation and/or recovery of a desired component.
It is well known that halocarbons of commercial interest including chlorofluorocarbons (CFC's), hydrogen-containing chlorofluorocarbons (HCFC's), hydrogen-containing fluorocarbons (HFC's) and perfluorocarbons (FC's) can be manufactured by processes involving halogen exchange reactions wherein a halocarbon which contains a halogen substituent (most often a chlorine substituent) other than or in addition to a fluorine substituent is reacted with a fluorine-donor compound. The fluorine-donor compound is capable of donating a fluorine atom in halogen exchange reactions such as a metal fluoride or hydrogen fluoride used in the presence of various catalytic compounds. The desired product is obtained together with by-products such as under- or over-fluorinated products, decomposition products, unreacted reactants and the like. The desired product usually must then be separated from these by-products and/or contaminants.
While most separations of this type are carried out readily by well known processes such as distillation, phase separation and the like, some separations present problems which preclude using these processes especially when halocarbons form azeotropic or azeotrope-like compositions with other compounds.
One example of this problem is in the manufacture of a fluoro-olefin, tetrafluoroethylene (CF2=CF2; TFE). TFE is an important component in the manufacture of TFE fluorocarbon resins. TFE is usually manufactured by the pyrolysis reaction of monochlorodifluoromethane (CHCIF2; HCFC-22), which reaction can be represented by the equation: ##STR1##
It is believed that in the above pyrolysis reaction, an intermediate, difluorocarbene, is formed which then dimerizes to form the desired TFE. However, difluorocarbene can also undergo an insertion reaction with TFE to form hexafluoropropylene (CF3--CF.dbd.CF2) and, thus, in this reaction, TFE is usually accompanied by some hexafluoropropylene. Hexafluoropropylene is a useful chemical and is important as a polymerizable fluoromonomer, as an intermediate for the preparation of fluorosurfactants and stable lubricants, and the like. The above pyrolysis reaction of HCFC-22 can also be written as: ##STR2##
TFE and hydrogen chloride can be separated from the reaction mixture by distillation but the separation of hexafluoropropylene from HCFC-22 presents a problem since hexafluoropropylene and HCFC-22 form an azeotropic mixture which contains about 15 mol percent hexafluoropropylene and about 85 tool percent HCFC-22 . As is well known, the separation of the components of an azeotropic or azeotrope-like mixture is difficult and often requires the addition of extraneous materials if a method such as extractive distillation is used. This usually creates additional problems of contamination and/or waste disposal.
Two other examples of the difficulties involved in separating azeotrope or azeotrope-like mixtures occur in the implementation of new regulations designed to protect the stratospheric ozone layer from possible damage by fully halogenated chlorofluorocarbons such as dichlorodifluoromethane (CFC-12). These regulations include rules requiring the replacement of such chlorofluorocarbons by fluorocarbons containing no chlorine, or by hydrogen-containing chlorofluorocarbons, i.e. hydrochlorofluorocarbons, which have a shorter life in the atmosphere. Other rules are aimed at promoting recovery and reuse of such compounds. The fluorocarbons especially useful as refrigerants, blowing agents, propellants and other uses include compounds such as 1,1,1,1-tetrafluoroethane (HFC- 134a) and 1,1,1-trifluoroethane (HFC-143a). In addition, difluoromethane (HFC-32) has been proposed as a replacement for various chlorofluorocarbons, both as a separate product and in blends with HFC-134a and/or other fluorocarbons. The hydrochlorofluorocarbons used for refrigerant and air conditioning applications include (among others) chlorodifluoromethane (HCFC-22), widely used for decades as a home air conditioning agent.
Difluoromethane may be prepared by the hydrodechlorination of chlorodifluoromethane and by other processes. In many of these processes, some carbon-carbon linkages may occur, leading to two-carbon impurities such as HFC-134a and HFC-143 a. The latter compound is particularly difficult to remove by conventional means such as distillation, since its boiling point differs from HFC-32 by only 4 degrees C.
A second result of the above regulations is the need to recycle existing refrigerants rather than allow them to escape into the atmosphere when equipment maintenance or refrigerant changes are required. We have found that, very often, the returned refrigerant will contain small amounts of impurities making them unsuitable for recycle. This is particularly a problem when HCFC-22 is returned containing a small mount of CFC-12 , since the two compounds form an azeotrope-like mixture not easily separated by distillation. This problem may occur frequently, since HCFC-22 and CFC-12 represent the two most widely used refrigerants and air conditioning agents in existing old equipment.
It is therefore an object of the present invention to provide a process for the separation of the components of azeotropic or azeotrope-like mixtures containing only organic compounds at least one of which compounds is a halocarbon. Another object is to provide an economical and efficient process to accomplish the above objective.
Important advantages of the instant invention is that the separation is achieved without the addition of extraneous materials, without altering any of the components of the mixture and without creating additional waste disposal problems.